Article pubs.acs.org/EF
Kinetic Study on the Process of Cyclopentane + Methane Hydrate Formation in NaCl Solution Qiunan Lv,†,‡,§ Yongchen Song,† and Xiaosen Li*,‡,§ †
Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian, Liaoning 116024, People’s Republic of China ‡ Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, and §Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, People’s Republic of China ABSTRACT: In this work, the formation kinetics of cyclopentane (CP) + methane hydrate is studied. CP is used as a promoter to accelerate the hydrate formation. The total methane consumption, the induction time, and the formation rate were investigated under different hydrate formation conditions in NaCl solution. The results indicated that the pressure driving force could increase the gas consumption and shorten the induction time. Meanwhile, the induction time could be greatly influenced by the pressure driving force at a lower temperature. Especially, it could be shortened to a minimum value of 110 s with the increase of the pressure driving force at a fixed operating condition (CP concentration, 7.45%; NaCl solution concentration, 3.50%; and temperature, 298.15 K). Moreover, the hydrate formation rate would be accelerated with the increase of the stirring rate by its promotion in the dissolution and dispersion of methane. Finally, a higher CP concentration was favorable for the rapid hydrate formation of CP + CH4 binary hydrates. The amount of CP used could determine the amount of methane incorporated into the hydrate phase.
1. INTRODUCTION Natural gas hydrates (NGHs) are non-stoichiometric crystalline, inclusion compounds, formed from water and small gas molecules at high pressures and low temperatures. A huge amount of natural gas hydrates occurs naturally within or under permafrost regions and ocean sediments, and 1 m3 hydrate of pure methane can be decomposed to produce up to 160−180 m3 of methane gas. Therefore, NGHs are considered to be a potential energy source in the 21st century with the traits of large reserves, high energy density, and wide-ranging distribution.1 To exploit this large energy resource, the researchers have proposed many methods, including depressurization,2,3 to reduce the reservoir pressure below the hydrate equilibrium pressure, thermal stimulation,4,5 to raise the temperature above the hydrate equilibrium temperature by injecting stream, hot water, or hot brine, chemical injection,6 to change the equilibrium hydrate decomposition conditions by injecting chemical additives, replacement of methane by CO2 hydrate,7 and the combination of these.8,9 Every method has advantages and disadvantages. The depressurization is demonstrated to be highly energy-efficient because there is no need for an external source of energy. However, it has a low production rate because of a long dissociation time, and the exploitation process is not stable and successive. The dissociation reaction is endothermic, and as a result, the temperature of the surrounding media or rock matrix significantly decreases. There is a possibility of gas hydrate reformation and produced water freezing. Thermal stimulation has the advantages of being more efficient and easier to control the exploitation process than the other methods. However, the heat loss is significant10 during the injection of steam or hot water from the ocean surface to the NGH region. It would significantly increase the production © 2016 American Chemical Society
costs. In comparison, chemical inhibitor stimulation is highly effective for hydrate dissociation,5,11−13 but it has the disadvantages of possibly being an environmental hazard and the high cost of the inhibitor used. Although the CO2-replacing method couples the advantages of the NGH production with CO2 emission, the CO2 hydrate formation may lead to a decrease of the permeability of the NGH deposits, and as a result, the production becomes very slow.14 We developed a novel technique to prepare hot brine in situ seafloor for marine NGH exploitation based on hydrate technology.15 A hydrate formation agent is introduced into an apparatus installed in situ seafloor, in which water and the hydrate formation agent form solid hydrates spontaneously under the ocean high-pressure conditions. The solid hydrates float up to the ocean surface by their own buoyancy. Because of the decrease in the pressure, the solid hydrates decompose into water and the hydrate formation agent, and the hydrate formation agent is recycled at the ocean surface. The unreacted brine is concentrated and heated during the formation of hydrate. The hot brine is injected into the NGH deposits for NGH dissociation. In comparison to the conventional injection thermal exploitation, this novel technique effectively avoid the heat loss during the transitioning of the heat medium. The hydrate formation agent must be nontoxic or low toxic and insoluble with seawater, and the density of the hydrate formation agent is lower than that of brine to facilitate its recovery and recycling. At the same time, the hydrate formation agent can accelerate the hydrate formation, and the hydrate formation enthalpy should be as high as possible. According to Received: November 10, 2015 Revised: December 28, 2015 Published: January 14, 2016 1310
DOI: 10.1021/acs.energyfuels.5b02634 Energy Fuels 2016, 30, 1310−1316
Article
Energy & Fuels
Figure 1. Schematic of the experimental apparatus.
the research indicated,16−20 cyclopentane (CP) is found to be an effective hydrate formation additive, which can accelerate the gas hydrate formation, and the hydrate formation enthalpy of CP + methane is as high as 130.25 kJ mol−1 at 285 K. The phase equilibrium conditions of CP hydrate were measured by Fan et al.21 It was found that it was difficult to form the hydrate of CP initially. Lo et al.22 studied the structure of water molecules at the water−CP hydrate interface, which is the same as that in the large cavity of a structure II (sII) hydrate. Sakemoto et al.23 observed CP hydrate crystal growth at the seawater and CP interface. Sun et al.24 studied the effect of CP on methane hydrate formation conditions and found that CP could reduce hydrate induction time and increase the gas hydrate formation rate, and formation pressure of methane hydrate was depressed greatly. Cai et al.25 investigated the kinetics of formation and growth of CP + methane hydrate at relatively warm temperatures (>15 °C) and moderate pressures (
